1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing a vitreous silica crucible, and more specifically relates to a method and an apparatus for manufacturing a vitreous silica crucible having an excellent inner surface property.
2. Description of the Related Art
A silicon single crystal has been manufactured by the Czochralski method (the CZ method) using a vitreous silica crucible. In this method, a seed crystal is dipped into a silicon melt in a vitreous silica crucible at high temperature, and the seed crystal is gradually pulled to produce a silicon single crystal. A highly-pure vitreous silica crucible is used to contain the silicon melt.
A vitreous silica crucible for pulling single crystal silicon is primarily manufactured by the arc fusing method. In this method, silica powder is deposited on an inner surface of a carbon-made rotating mold to form a silica powder layer with a predetermined thickness, and the silica powder layer is heated and fused to be vitrified by arc discharge generated by electrodes mounted on an upper position of the inner side of the mold. This method is called “the rotating mold method.”
This vitreous silica crucible has a two-layer structure including a vitreous silica layer containing a number of bubbles on the outer side, and a transparent layer vitreous silica layer on the inner side (hereinafter, referred to as “transparent layer”). Such vitreous silica crucible is manufactured by a method including the process of fusing silica powder layer under a reduced pressure, which is caused by suction from the mold side (JP-A-H6-191986 and JP-A-H10-025184). In the fusing process, in order to achieve deaeration of the silica powder layer by the reduced pressure (this is called “vacuum suction”), a thin vitreous silica layer is formed by uniformly fusing an inner surface of the silica powder layer. This thin vitreous silica layer (hereinafter referred to as “seal layer”) seals the inner surface to enhance the vacuum level of the inside of the silica powder layer.
Nowadays, wafers with a diameter of over 300 mm have been manufactured in order to enhance efficiency of a device manufacturing process, and this requires longer pulling time. Therefore, there is required a vitreous silica crucible, which can pull a single crystal with a large diameter at a temperature of over 1400 degrees C. and for a period of approximately 100 hours. In addition, improvement of crucible properties such as an inner surface state etc. of a vitreous silica crucible has been strongly demanded because such crucible properties directly influence controllability of oxygen concentration Oi and single crystallization yield, etc., which are demanded for device miniaturization.